chore: update action configuration for greater flexibility (#240)

- Update base image version from `1.6.12` to `1.6.13` in Dockerfile
- Add `envs_format` input with flexible configuration in action.yml
- Add a line to README.md pointing to action.yml for more information

fix https://github.com/appleboy/ssh-action/issues/213

action.yml

@@ -65,6 +65,8 @@ inputs:
     default: false
   envs:
     description: 'pass environment variable to shell script'
+  envs_format:
+    description: 'flexible configuration of environment value transfer'
   debug:
     description: 'enable debug mode'
     default: false